(a) Field of the Invention
The present invention relates to a surface-treated steel sheet for fuel tanks and a method of fabricating the same and, more particularly, to a surface-treated steel sheet which is well adapted for use in forming fuel tanks while exhibiting good performance characteristics in chemical resistance, corrosion resistance and weldability.
(b) Description of the Related Art
Generally, it is required that a steel sheet for fuel tanks should has resistance to corrosion on its outer surface to be exposed to the atmosphere (hereinafter referred to as the xe2x80x9ccosmetic corrosion resistancexe2x80x9d) as well as resistance to corrosion on its inner surface to contact fuel such as gasoline (hereinafter referred to as the xe2x80x9cfuel corrosion resistancexe2x80x9d).
A fuel tank is usually made by press-forming steel sheets into cup-shaped upper and lower tank bodies, and welding the bodies to each other by spot welding, seam welding, soldering, or brazing. In this respect, good weldability is also required for the steel sheet to be applied for use in forming fuel tanks.
A ternesheet that is a lead-tin (Pbxe2x80x94Sn) alloy plated steel sheet has widly used as such a steel sheet for fuel tanks. However, the ternesheet should have been limited in its use because it contains lead (Pb) harmful to the human body. In this connection, extensive researches have been made to develop a surface-treated steel sheet for fuel tanks without any lead content.
Japanese Patent Laid Open Publication No. Sho63-19981 discloses a surface-treated steel sheet where a steel sheet is overlaid with a zinc (Zn) plating layer and a chromate film. However, such a chromate film has poor fuel corrosion resistance so that the zinc content of the zinc plating layer elutes and generates white rusts. The white rusts are floating in fuel and cloak the fluid flowing paths such as a filter.
Japanese Patent Laid Open Publication Nos. Sho63-69361 and Hei2-18982 disclose another kind of surface-treated steel sheet where a steel sheet is overlaid with a zinc or zinc-based alloy (Znxe2x80x94Ni, Znxe2x80x94Co, Znxe2x80x94Fe or Znxe2x80x94Al) plating layer, and an organic resin coating layer. The organic resin coating layer is formed with phenoxy resin, epoxy and metallic powder. The amount of deposition of the zinc or zinc-based alloy is 200 g/m2 and that of the organic resin is 50 xcexcm. As such large amount of deposition causes for the resulting resin coating layer to be too thick, adherence of the resin coating layer to the plating layer is weakened so that they are liable to be desquamated from each other. Furthermore, such a structure is not cost effective while bearing poor chemical resistance and poor corrosion resistance.
Korean Patent Application No. 97-703448 and Japanese Patent Laid Open Publication No. Hei9-59783 disclose still another kind of surface-treated steel sheet where a steel sheet is overlaid with a zinc-nickel (Znxe2x80x94Ni) alloy plating layer and a chromate film. The chromate film is formed from a chromate solution containing resin and silica. Minute cracks are formed on the zinc-nickel alloy plating layer to enhance corrosion resistance, but such crack formation bears complicated processing steps. Furthermore, in such a structure, the chrome content is liable to elute even in contact with minimum amount of water to be contained in fuel, and this results in deteriorated fuel corrosion resistance.
Accordingly, there is a need for developing a surface-treated steel sheet for fuel tanks that satisfy all the requirements of weldability, formability, cosmetic corrosion resistance, and fuel corrosion resistance at the same time.
It is an object of the present invention to provide a surface-treated steel sheet which is well adapted for use in forming fuel tanks while exhibiting good physicochemical characteristics.
This and other objects may be achieved by a surface-treated steel sheet including a cold-rolled steel sheet with a low carbon content, a zinc or zinc-based alloy plating layer formed on the steel sheet, and a chromate film coated on the zinc or zinc-based plating layer. The chromate film is formed from a chromate solution. The chromate solution includes a subject solution containing a chrome aqueous solution where the ratio of trivalent chrome to the chrome content is in the range of 0.4-0.8 and the concentration of chrome is in the range of 7-50 g/l. Phosphoric acid in an amount ranging from 20 to 150% by weight with respect to the chrome content, fluoric acid in an amount ranging from 10 to 100% by weight with respect to the chrome content, colloidal silica having pH of 2-5 in an amount ranging from 50 to 2000% by weight with respect to the chrome content, and sulfuric acid in an amount ranging from 5 to 30% by weight with respect to the chrome content are mixed with the chrome aqueous solution. An aqueous solution in an amount ranging from 5 to 50% by weight with respect to the subject solution is added to the subject solution. The aqueous solution contains Epoxy-based silane in an amount ranging from 5 to 50% by weight with respect to the hardening solution and has a pH of 2-3. The amount of chrome (Cr) in the chromate film is in the range of 20-250 mg/m2.
A resin coating layer may be formed on one side or both sides of the chromate film. The resin coating layer is formed from a resin solution. The resin solution includes a phenoxy resin solution having a molecular weight of 25,000-50,000, colloidal silica of 10-20 phr with respect to the phenoxy resin content, and melamine resin of 2-15 phr with respect to the phenoxy resin content.
The chromate film and the resin coating layer are all to improve cosmetic corrosion resistance and fuel corrosion resistance of the surface-treated steel sheet. With the addition of appropriate amount of para toluene sulfonic acid (p-TSA), wax and metallic powder to phenoxy resin, the physicochemical characteristics of the surface-treated steel sheet may be further improved.